Skin protects the body's organs from external environmental threats and acts as a thermostat to maintain body temperature. It consists of several different layers, each with specialized functions. The major layers include the epidermis, the dermis and the hypodermis. The epidermis is a stratifying layer of epithelial cells that overlies the dermis, which consists of connective tissue. Both the epidermis and the dermis are further supported by the hypodermis, an internal layer of adipose tissue.
The epidermis, the topmost layer of skin, is only 0.1 to 1.5 millimeters thick (Inlander, Skin, New York, N.Y.: People's Medical Society, 1-7 (1998)). It consists of keratinocytes and is divided into several layers based on their state of differentiation. The epidermis can be further classified into the stratum corneum and the viable epidermis, which consists of the granular melphigian and basal cells. The stratum corneum is hygroscopic and requires at least 10% moisture by weight to maintain its flexibility and softness. The hygroscopicity is attributable in part to the water-holding capacity of keratin. When the horny layer loses its softness and flexibility it becomes rough and brittle, resulting in dry skin.
The dermis, which lies just beneath the epidermis, is 1.5 to 4 millimeters thick. It is the thickest of the three layers of the skin. In addition, the dermis is also home to most of the skin's structures, including sweat and oil glands (which secrete substances through openings in the skin called pores, or comedos), hair follicles, nerve endings, and blood and lymph vessels (Inlander, Skin, New York, N.Y.: People's Medical Society, 1-7 (1998)). However, the main components of the dermis are collagen and elastin.
The hypodermis is the deepest layer of the skin. It acts both as an insulator for body heat conservation and as a shock absorber for organ protection (Inlander, Skin, New York, N.Y.: People's Medical Society, 1-7 (1998)). In addition, the hypodermis also stores fat for energy reserves. The pH of skin is normally between 5 and 6. This acidity is due to the presence of amphoteric amino acids, lactic acid, and fatty acids from the secretions of the sebaceous glands. The term “acid mantle” refers to the presence of the water-soluble substances on most regions of the skin. The buffering capacity of the skin is due in part to these secretions stored in the skin's horny layer.
Wrinkles, one of the telltale signs of aging, can be caused by biochemical, histological, and physiologic changes that accumulate from environmental damage (Benedetto, International Journal of Dermatology, 38:641-655 (1999)). In addition, there are other secondary factors that can cause characteristic folds, furrows, and creases of facial wrinkles (Stegman et al., The Skin of the Aging Face Cosmetic Dermatological Surgery, 2nd ed., St. Louis, Mo.: Mosby Year Book: 5-15 (1990)). These secondary factors include the constant pull of gravity, frequent and constant positional pressure on the skin (i.e., during sleep), and repeated facial movements caused by the contraction of facial muscles (Stegman et al., The Skin of the Aging Face Cosmetic Dermatological Surgery, 2nd ed., St. Louis, Mo.: Mosby Year Book: 5-15 (1990)). Different techniques have been utilized in order potentially to mollify some of the signs of aging. These techniques range from facial moisturizers containing alpha hydroxy acids and retinol to surgical procedures and injections of neurotoxins.
One of the principal functions of skin is to provide a barrier to the transportation of water and substances potentially harmful to normal homeostasis. The body would rapidly dehydrate without a tough, semi-permeable skin. The skin helps to prevent the entry of harmful substances into the body. Although most substances cannot penetrate the barrier, a number of strategies have been developed to selectively increase the permeability of skin with variable success.
Since non-protein non-nucleotide therapeutic agent such as antifungals cannot penetrate the skin efficiently, in order to provide the therapeutic effects antifungal agents, it must currently be injected into the skin or administered systemically. The Federal Food and Drug Administration has approved such a procedure, for treatment of fungal infection. In such treatments, the antifungal agent is administered by monitored injection or dosage. However, such treatment can be cause adverse side effects. Topical application of antifingal agents provides a local delivery for a safer and more desirable treatment alternative due to painless nature of application, reduced training to apply the antifungal therapeutic, smaller doses necessary to affect and to reach a therapeutic clinical result and limiting side effects typically associated with systemic delivery.
Since antigenic agents suitable for immunization cannot penetrate the skin efficiently, in order to provide the therapeutic effects of antigenic agents suitable for immunization the toxin must currently be injected into the skin. The Federal Food and Drug Administration has approved such a procedure, for treatment of for example, malaria, rabies, anthrax, tuberculosis, or related to childhood immunizations such as hepatitis B, diptheria, pertussis, tetanus, Haemophilus influenza type b, inactivated poliovirus, measles, mumps, rubella, varicella, pneumococcus, hepatitis A, and influenza. In such treatments, the antigenic agent for immunization is administered by monitored injection. However, such treatment can be uncomfortable and more typically involves some pain. Topical application of antigenic agent for immunization provides for a safer and more desirable treatment alternative due to painless nature of application, the larger treatment surface area that can be covered, reduced training to apply the therapeutic, smaller doses necessary to affect and to reach a therapeutic clinical result.
Transdermal administration of other therapeutics is also an area of great interest due, for instance, to the potential for decreased patient discomfort, direct administration of therapeutic agents into the bloodstream, and the opportunities for monitored delivery via the use of specially constructed devices and/or of controlled release formulations and techniques.